DESCRIPTION: (Applicant's Abstract) Marijuana is the most commonly abused illegal drug in the United States. It is of particular concern that this drug is abused by large numbers of adolescents. Thus, the acute and chronic effects of marijuana and its active ingredients, including THC, are of considerable research interest and clinical significance. A number of investigators demonstrated alterations in brain function via neuropsychological testing in chronic marijuana users. With the recent advent of non-invasive techniques, it has become possible to measure cerebral blood flow (CBF) during cognitive testing in humans. In such activation studies, CBF serves as an index of regional brain function. All four available studies on CBF in chronic marijuana smokers found reduced global flow which reached statistical significance in three. Studies were conducted with the 133 xenon inhalation technique and positron emission tomography. Acute marijuana intoxication was found to increase CBF with more marked changes in the right hemisphere and in the cingulate gyrus. Behavioral studies have indicated a number of neuropsychological abnormalities, including impairment of short-term memory as measured with the digit symbol substitution test. In the proposed study we shall use PET to assess regional CBF changes during recognition and recall activation tasks following THC or placebo infusion in marijuana smokers. We shall also compare resting CBF with neuropsychological test.